WO2001094905A1

WO2001094905A1 - Measuring method for detecting non-linearities of an optical fiber

Info

Publication number: WO2001094905A1
Application number: PCT/DE2001/001975
Authority: WO
Inventors: Christoph Glingener; Erich Gottwald
Original assignee: Siemens Aktiengesellschaft
Priority date: 2000-06-07
Filing date: 2001-05-22
Publication date: 2001-12-13
Also published as: DE10028144C1; US20030128946A1; US6819412B2; EP1297316A1

Abstract

The invention relates to a measuring method that allows to measure non-linearities of an optical fiber (OF) unilaterally, that is at the beginning or at the end of the optical fiber (OF). To this end, an optical test signal (ots) is launched into the optical fiber (OF), the test signal power (PS) of which is modified. A first entry threshold (SBS1) of the stimulated Brillouin scattering is detected on the basis of the modified power (Pros) of the reflected optical signal (ros). In a second step, at least one modulated optical pump signal (ops) is launched into the optical fiber (OF) in addition to the optical test signal (ots). A second entry threshold (SBS2) of the stimulated Brillouin scattering is detected on the basis of the modified power (PS) of the optical test signal (ots). The non-linearity coefficient ( gamma ) of the optical fiber (OF) is determined by evaluating the at least first and second entry thresholds (SBS1, SBS2), the test and pump signal parameters and the fiber parameters.

Description

description

Measuring method for determining the non-linearities of an optical fiber

The invention relates to a measuring method for determining the nonlinearities of an optical fiber.

In optical transmission systems, in particular in transmission systems operating according to the DM principle (Wavelength Division Multiplexing), nonlinear effects, for example self-phase modulation, cross-phase modulation and four-wave mixing, are known, caused by the signal distortions of the optical signal to be transmitted in the optical fiber become. Such nonlinear effects in an optical fiber can be described by the nonlinearity coefficient.

To determine the nonlinearity coefficient of an optical fiber see, for example, the publication by Y. Namihira, A. Miyata, N. Tanahashi, "Nonlinear coefficient measurements for dispersion shifted fibers using self-phase modulation method at 1.55 μm", Electronic Letters, 1994 , Vol. 30, No. 14, pp. 1171-1172 discloses a measuring arrangement in which the non-linearity properties of an optical fiber are determined by using the self-phase modulation method. Such measurement methods require access to the beginning and end of the optical fibers to be measured, which, however, is necessary for existing optical communication networks, i.e. with already installed optical fibers, a considerable one

Measurement effort required or is almost impossible in individual cases. In addition, a separate return channel from the fiber end to the fiber start is required to transmit the measured information.

The object on which the invention is based is to determine the non-linearities of an optical fiber improve or enable a measurement of the non-linearities of an optical fiber carried out on one side, ie at the beginning or at the end of the optical fiber. The object is achieved by a measuring method according to claim 1 by the features of the characterizing part.

The essential aspect of the measuring method according to the invention is to be seen in a first step in which at least one optical test signal is coupled into the optical fiber, the test signal power of which is changed and a first entry threshold of the stimulated Brillouin scattering is determined on the basis of the change in the power of the backscattered optical signal. Furthermore, in a second step, in addition to the optical test signal, at least one modulated optical pump signal with a predetermined pump signal power and a first pump wavelength is coupled into the optical fiber and a second entry threshold of the stimulated Brillouin scatter is determined on the basis of the change in the power of the optical test signal. Finally, the non-linearity coefficient of the optical fiber is determined by evaluating at least the first and second entry thresholds, the test and pump signal parameters and the fiber parameters. With the aid of the measuring method according to the invention, a determination of the non-linearity coefficient only by a one-sided, i.e. on the receiving side or on the transmitting side, measurement possible. This is of enormous advantage in particular when determining the fiber non-linearities of already laid optical fibers.

In a second variant of the measuring method for determining the non-linearities in an optical fiber, at least one optical test signal with a test signal power and a test signal wavelength is coupled into the optical fiber in a first step and the power of the backscattered optical signal is measured, as well as a first ratio of the coupled test signal power and the power of the backscattered optical signal. Furthermore, in a second step in addition to the optical test signal having a test signal power and test wavelength, coupling at least one modulated optical pump signal with adjustable pump signal power and a first pump wavelength into the optical fiber and measuring the power of the backscattered optical signal and a second ratio of the coupled test signal power and the power of the backscattered optical signal is determined. The adjustable pump signal power of the modulated optical pump signal is increased or decreased until the second ratio matches the first ratio. The non-linearity coefficient of the optical fiber is then determined by evaluating the test and pump signal parameters as well as the fiber parameters. The variation of the pump signal power of the modulated optical pump signal according to the invention alternatively makes the non-linearity coefficient of the optical by evaluating the existing fiber parameters and test parameters Fiber determinable.

Another advantage of the measurement method according to the invention is that the test and pump signal parameters evaluated according to the first variant of the measurement method according to the invention provide the test signal wavelength, the predetermined pump signal power, the first pump wavelength and the modulation frequency of the optical pump signal Furthermore, the test signal power, the test signal wavelength, the set pump signal power, the first pump wavelength, the modulation frequency of the optical pump signal are evaluated as the test and pump signal parameters relevant for the second variant of the measuring method according to the invention.

Advantageous further developments and developments of the measuring method according to the invention are described in the further patent claims. Theoretical foundations for the measurement method according to the invention for determining the non-linearities and the dispersion in an optical fiber are explained below.

Depending on the input power of an optical test signal or signal, the nonlinear effect of "stimulated Brillouin scattering (SBS)", ie the stimulated Brillouin scattering, forms in optical fibers. This narrow-band effect of SBS with a line width of Δv _B <25 MHz due to the phonon lifetime is known - see Govind P. Algrawal "Nonlinear Fiber Optics", Academic Press, 1995, pages 370 to 375. Furthermore, from US Pat. No. 3,705,992 discloses that the entry threshold of the SBS increases in proportion to the ratio of the spectral width Δv _s of the optical signal coupled into the optical fiber to the line width Δv _B , ie

with IS _B S - intensity of the injected optical signal at the SBS entry threshold

The energy that is spectrally integrated in a frequency interval of width Δv _B is decisive for reaching the SBS entry threshold. In standard single-mode fibers, the SBS entry threshold, for example for unmodulated optical signals or test signals, is approximately below 10 mW and for binary amplitude-modulated optical signals by a factor of 2 to 3 dB higher. The increase in the case of the binary amplitude-modulated optical signal can be attributed to the distribution of the optical signal power between modulation sidebands and carrier signals, especially since the power of the data signal is distributed over a broad spectral band, in particular at data rates in the Gbit / s range. In the case of amplitude-modulated signals, the SBS leads to signal distortion due to overmodulation, see in particular H.Kawakani, "Overmodulation of Intensity modulated Signals due to stimulated Brillouin scattering", Electronic Letters, Vol. 30, No. 18, pages 1507 to 1508, since essentially the carrier of the amplitude-modulated optical signal, in which the spectral energy density with chip-free modulation is identical to the laser light source, experiences a strong additional attenuation.

The SBS entry threshold can be increased considerably by significantly reducing the integrated energy spectral density of the optical signal over a frequency band of width Δv _B. In the case of amplitude-modulated optical signals, the carrier signal power, measured with a resolution Δv _B , should therefore be reduced to values well below the SBS threshold power. Such a reduction can be achieved by frequency or phase modulation.

The SBS effects in the optical fiber essentially take place with a standard single-mode fiber within the first 20 km (effective length L _eff ). The optical signal takes the time to pass through the effective length L _eff :

^L eff ^'n

T = (= 0.1 ms for L _eff = 20 km)

C

To reduce SBS effects, the optical input power per frequency interval Δv _B averaged over a time interval should be much smaller than the transit time τ below the SBS threshold power. In the case of SBS suppression by frequency modulation or amplitude modulation, the required relationship between the modulation stroke and the modulation frequency for different forms of modulation can be derived from this requirement.

In order to reduce the spectrally narrow carrier line of the optical signal and to evenly distribute its power over as many lines as possible arising from the phase modulation with a frequency spacing greater than Δv _B , the Phase modulation thus takes place with modulation frequencies> Δv _B. With increasing phase shift, ie modulation index m = 4Λ

J m with Δf _p = peak frequency deviation; f _ra = modulation frequency;

the spectral power decreases per frequency interval. Such an amplitude modulation in the optical fiber can be caused, for example, by the non-linear effect of cross-phase modulation (XPM) by an additional coupling in of strongly amplitude-modulated pump signals in addition to the optical signals. Here, the phase modulation along the optical fiber caused by the cross-phase modulation (XPM) shows an RC low-pass behavior. The cut-off frequency ω _{g of} the "low-pass behavior" decreases linearly with increasing channel spacing due to the dispersion-related slip of the WDM transmission channels. In order to achieve an effectively effective phase modulation over a broad wavelength band with the help of the XPM, the height of the modulation frequency must therefore be chosen to be as low as possible, although this should in no case be below the line width Δv _B.

The intensity I _{SBS of} the backscattered optical signal by the SBS at the beginning of the fiber increases in the reverse direction with increasing coupled optical signal power according to the following exponential relationship - see Govind P. Agrawal "Nonlinear Fiber Optics", 1995, chapter 9.2.1:

^J SB _S (θ) = Isas (z) ^■ exp (g ₅ ^■ I _s ^■ L _ejf -a ^■ z) (Al)

with L _eff = - (l - exp (-α - z)) (A-2) a •

With simultaneous propagation of the amplitude-modulated optical pump signal causing the XPM and the optically see signal in the fiber, the optical signal is increasingly phase-modulated due to the XPM with increasing distance. The phase modulation, for example with a phase shift of 1,435 rad, distributes the spectral power of the carrier signal over several frequencies, ie, for example, evenly over the carrier wave and the two first sidebands. If the modulation frequency is greater than the SBS line width Δv _B , only about 1/3 of the spectral energy density is available for the formation of the SBS, for example, ie the SBS entry threshold increases by a factor of 3 from the location at which such phase shift is achieved by the XPM. The local SBS entry threshold can thus be calculated as a function of the properties of the injected modulated pump signal and the optical fiber and the injected optical signal, and the resulting SBS entry threshold dependent on the optical pump signal can be determined for the entire fiber.

The calculation of the SBS entry threshold in the presence of the optical pump signal is carried out by splitting the fiber into small sections in combination with Eq. (A-l) realized. In a rough approximation, the fiber is first broken down into n = 2 sections, from which follows with Eq. (A-l) and (A-2) for a fiber of length z / 2:

ISBS (Z / 2) = I _SBS (z) * exp (g _B * I _s * exp (-α * z / 2) * l / α * (1-exp (-α * z / 2)) - αz / 2) (A-3)

and ISBS (0) = I _SBS (z / 2) * exp (g _B * I _s * 1 / α * (l-exp (-α * z / 2)) - αz / 2) (A-

The following applies to a breakdown into n sections:

exp (-α * k / n * z)} * l / α * (1-exp (-

α * z / n)) - αz] (A-5) The second section of the route - equation (A-3) - is considered below. Taking into account the spectral change in the optical signal I _s by the XPM, which is induced in the fiber by a sinusoidally amplitude-modulated optical pump signal I _P , there is a further additional attenuation in addition to the path attenuation exp (-α * z / 2) for the wearer with the damping factor:

Jo ² (m) = J ₀ ² (ξ * γ * Ip * l / α * (l-exp (-α * z / 2)), (A-6)

where m denotes the phase shift or modulation index caused by the XPM on the first section of length z / 2 and ξ represents a polarization-dependent constant. If the polarization varies randomly: ξ = 8/9

Investigations show that essentially the carrier of the amplitude-modulated optical pump signal (thus J ₀ ² (m)) is included in the change in the intensity of the backscattered optical signal. For equation (A-3) with (A- 6) it follows:

Is _B s (z / 2) = I _S Bs (z) * exp [g _B * Is * l / α * (l-exp (-α * z / 2)) * exp (-α * z / 2) * o ² (m (z / 2)) - αz / 2] (A-7)

with: m (x) = ξ * γ * I _P * l / α * <l-exp (-α * x)) (A-8)

Equation (A-7) inserted in equation (A-4) provides the intensity of the backscattered optical signal I _SBS , taking the XPM into account.

ISBS (0) = I _SBS (z) * exp [g _B * I _s * exp (-α * z / 2) * J ₀ ² (m (z / 2)) * l / α * * (l-exp (-α * z / 2)) * l / α * (l-exp (-α * z / 2)) - αz / 2] * * exp [g _B * I _s * 1 / α * (1-exp (-α * z / 2)) - αz / 2] =

= I _S Bs (z) * exp [g _B * Is * l / α * (l-exp (- * z / 2)) * * {1+ exρ (-α * z / 2) * J ₀ ² ( m (z / 2))} - αz] To increase the accuracy, the fiber is broken down into n sections (equation (A-5)), and the result is an analogous procedure:

I _SBS (0) = I _SBS (z) * exp [g _B * I _s * l / α * (l-exp (-α * z / n)) * (A-9) nl

* {1+ ∑ exp (-α * k / n * z) * J ₀ ² (m (k * z / n))} - αz] i = l

Comparison of equation (A-9) with equation (A-l) shows that

L _eff = 1 / α * (l-exp (-α * z)) by the expression

H-l

L _eff (z, α, γ, I _P ) = 1 / α * (l-exp (-α * z / n)) * {1+ T exp (-α * k / n * z;

* = 1 J ₀ ² (m (kz / n))} (A-10)

can be replaced. The effective length L _eff is, according to equations (A-10) and (A-8), dependent on the non-linearity coefficient γ of the optical fiber and the optical power of the amplitude-modulated optical pump signal I _P.

Consideration of the dispersion

In the case of a large frequency spacing between the optical pump signal and the coupled-in optical test signals or signals, further dependencies between the effective length L _eff and that occur in particular due to the dispersion-related slip of the optical signal and the optical pump signal that occur in a standard single-mode fiber (SSMF) Fiber dispersion, the frequency spacing (wavelength spacing) of the optical pump signal and optical test signal or the modulation frequency of the optical pump signal.

From equation (A-10) we get for L _eff (z, α, γ, I _P ) for a section z consisting of n sections:

Leff (z, α, γ, I _P , D, Δλ, fmod) = 1 / α * (1-exp (-α * z / n)) * * {1 ₊ exp (-α * k / n * z) * Jo ² (m (kz / n) * Le (k * z / n, α, D, Δλ, f od))} k = l

(A-ll) with Le (k * z / n, α, D, Δλ, fmod)

with: L = k * z / n, ß = D * Δλ, ω = 2 * π * fmod; m (kz / n) = ξ * γ * I _P * Le;

Le (k * z / n, α, D, Δλ, fmod) describes the variation of the modulation index {m (kz / n)} inter alia as a function of the modulation frequency and the wavelength distance between the optical pump signal and the test signal.

For high dispersion values D, high modulation frequencies fmod and large wavelength spacing Δλ, L _eff returns to its original form from equation (A-2), ie L _e f is only dependent on the fiber attenuation α and the location z and the one caused by the optical pump signal SBS suppression is reduced.

The variation of the SBS entry threshold as a function of the pump, signal and fiber parameters results from inserting equation (A-10) or equation (A-ll) in:

P _SBS = 21 * A _eff / g _B / Leff (A-12)

From Godvind P. Agrawal, "Nonlinear Fiber Optics", Academic Press, 1995, formula (9.2.6).

From the variation of the effective length L _e ff (z, α, I _P , D, Δλ, fmod) as a function of optical pump power I _P , the wavelength difference between pump and test signal Δλ and the modulation frequency fmod and the SBS entry threshold P _SBΞ can The dispersion D and the non-linearity coefficient γ can be determined from equation (A-II) and equation (A-12).

According to the measurement method according to the invention, the displacement of the SBS entry threshold P _{SBS is} determined by the change in the spectrum of the coupled-in optical test signal, which is caused by the cross-phase modulation (XPM) resulting from the sinusoidally amplitude-modulated optical pump signal in the optical fiber, to determine the non-linearity coefficient γ and the dispersion D. Equation (A-12) is used.

The Brillouin Gain constant g _B and the effective area A _{eff are} optical fiber constants, which are naturally available for the optical fiber to be measured or can be determined without considerable technical effort. However - as already mentioned - the effective length L _eff (z, α, I _P , D, Δλ, fmod) can be influenced by the test conditions and, according to formula (A-II), depends on the length of the fiber z, the fiber attenuation α, from the wavelength difference between the optical pump and test signal Δλ and the modulation frequency fmod of the amplitude-modulated optical pump signal.

In the measurement method according to the invention for determining the

Nonlinearity coefficient γ of an optical fiber can with a wavelength difference between the optical pump and test signal Δλ, for example less than 1 n and a modulation frequency fmod of the amplitude-modulated optical pump signal less than 200 MHz, the influence of dispersion in

With regard to the measurement result are neglected, ie the effective length L _eff does not depend on the fiber dispersion D in a first approximation. According to the invention, a first SBS entry threshold P _SBS ι and a second SBS entry threshold P _SB S2, which is modulated due to the input

Pump signal in the optical fiber-induced cross-phase modulation (XPM) is shifted, measured and put together evaluated with the test and pump signal parameters and the fiber parameters according to equations (A-II) and (A-12), the dispersion D being negligible. Alternatively, according to the invention, a first and a second measurement of the backscattered optical power can be carried out, with only the optical test signal with a predetermined power and wavelength being coupled into the optical fiber in the first measurement, and in addition to the optical test signal with the second measurement modulated optical pump signal for generating cross-phase modulation (XPM) is coupled into the optical fiber. In the first as well as in the second measurement, the coupled power of the optical test signal is increased until there is a predetermined ratio of the coupled power of the optical test signal and the backscattered power. The powers of the optical test signal and the optical pump signal set together with the test and pump signal parameters and the fiber parameters in the measurement method for the first and second measurement are in turn evaluated according to equations (A-II) and (A-12), the dispersion D is negligible.

If, in addition to determining the non-linearity coefficient γ, the dispersion constant D of an optical fiber is to be determined with the aid of the measurement method according to the invention, the wavelength difference between the optical pump and test signal Δλ is chosen, for example, to be greater than 10 nm, ie the effective length L _eff depends on the fiber dispersion D. and the wavelength difference between the optical pump and test signal Δλ. According to the measurement method according to the invention, in addition to the first SBS entry _threshold P _SBsι - without a coupled-in modulated optical pump signal - a third SBS entry _threshold P _SBS3, which runs differently to the second SBS entry _threshold P _SBS2 due to the modified pump _{signal parameters, is} determined or with the aid of a third measurement which, in addition to the optical test signal, the modified modulated optical pump signal for generating the Cross-phase modulation (XPM) is injected into the optical fiber, the injected power of the optical test signal is increased until there is a predetermined ratio of the injected power of the optical test signal and the backscattered power. The detailed course of the measurement method according to the invention and the determination of the non-linearity constant γ and the dispersion constant D is explained in more detail with the aid of the following exemplary embodiment.

Show this

1 shows an example of a measuring arrangement for realizing the measuring method according to the invention and

FIG. 2 shows a shift of the SBS entry thresholds according to the invention, and

FIG. 3 in a further calculated diagram the measurement method according to the invention for determining the non-linearity constants and dispersion constants.

FIG. 1 shows a measuring arrangement MAO for implementing the measuring method according to the invention for determining the non-linearities of an optical fiber OF with the aid of a block diagram, an optical standard single-mode fiber OF being selected as the test object in FIG. 1 as an example. The measuring arrangement MAO shown in FIG. 1 has a test signal unit TSU, a pump signal unit PSU, an optical coupler OK, a controllable switching unit S, a circulator Z, a filter unit FU, a measuring transducer MW and a control and evaluation unit CU. The test signal unit TSU has a control input ri and a signal output e which is connected to the circulator Z via the optical coupler OK and via the first optical connecting line VL1. The optical coupler OK is in turn connected to the output e of the controllable switching unit S via a second optical connecting line VL2. The circulator Z is additionally connected via a third optical connecting line VL3 to the test object - the optical fiber OF - and 1975

14, a first feed line ZL1 is connected to the input fi of the filter unit FU, the output fe of which is connected to the input i of the transducer MW via a second feed line ZL2. The output e of the measuring transducer MW is connected via an electrical supply line EZL to the control and evaluation unit CU, which via a control line SL with control input si of the controllable switching unit S, via a first control line RL1 with the control input ri of the test signal unit TSU and via a second Control line RL2 is connected to the control input ri of the pump signal unit PSU. The pump signal unit PSU further has a first and second output el, e2, which are connected to the first and second input il, i2 of the controllable switching unit S. Instead of the circulator Z, for example an optical coupler OK can be used - not shown in FIG. 1.

A first power controller RL1, a second power controller LR2, a switching controller SR, a memory unit MEM, an evaluation unit AE and a control unit MC — for example implemented in a microprocessor — are provided in the control and evaluation unit CU. The memory unit MEM, the first and second power controllers RL1, RL2, the switching controller SR and the evaluation unit AE are connected to the control unit MC, the first and second power controllers LR1, LR2 being additionally connected to the evaluation unit AE. In addition, the first power controller LR1 via the first control line RL1 with the test signal unit TSU, the second power controller LR2 via the second control line RL2 with the pump signal unit PSU, and the switching controller SR via the switching line SL with the controllable switching unit S and the evaluation unit AE via the electrical supply line EZL connected to the measuring transducer MW.

The measurement method according to the invention is implemented, for example, on the basis of a measurement routine MR and an evaluation routine BWR in the control unit MC, which include the test signal unit TSU, the pump signal unit PSU and the control unit. 1975

15 bare switching unit S regulate or control. Thus, an optical test signal ots with a predetermined first wavelength λl and a predetermined power P _{s is} generated in the test signal unit TSU, wherein the optical test signal ots can additionally be amplitude-modulated, for example, with a first modulation frequency vl. According to the invention, the optical test signal ots is thus modulated or unmodulated into the test object, that is, into the optical fiber OF. In the measuring arrangement MAO shown in FIG. 1, the optical test signal ots is transmitted to the circulator Z, for example via the optical coupler OK and via the first distributor line VL1, and is coupled into the optical fiber OF by the circulator Z via the third distributor line VL3. For the first step of the method according to the invention, only the optical test signal ots is coupled into the optical fiber OF, ie none of the optical pump signals ops generated in the pump signal unit PSU is switched through to the optical coupler OK by the controllable switching unit S. This means that the third, unassigned input 13 of the controllable switching module S is switched through to the output e by the controllable switching unit S with the aid of a control command ss generated in the control controller SR.

In the optical fiber OF, depending on the injected test signal power P _{s of} the optical test signal ots, the nonlinear effect of the "stimulated Brillouin scattering (SBS)", ie the stimulated Brillouin scattering, is formed. This narrow-band, non-linear effect of the SBS causes part of the optical test signal ots to be backscattered or reflected in the opposite direction to the coupling direction. This backscattered optical signal ros is fed via the circulator Z and via the first feed line ZL1 to the filter input fi of the filter unit FU. In the filter unit FU, for example a bandpass filter with a narrow passband around the first wavelength λl of the optical test signal ots, the backscattered optical signal ros is filtered and the filtered backscattered signal largely emitted at filter output fe. The filtered backscattered signal is then largely transmitted via the second feed line ZL2 to the input i of the measuring transducer MW, for example an opto-electrical transducer, and converted into an electrical signal using the measuring transducer MW. The electrical signal is fed to the control and evaluation unit CU or the evaluation unit AE via the electrical feed line EZL, in which the power P _{ros of} the electrical signal es and thus of the backscattered optical signal ros is determined or evaluated.

The power P _{ros of} the backscattered optical signal ros is determined by the evaluation unit AE controlled by the control unit MC and the determined backscattered signal power P _{ros is} compared with the power P _{s of} the optical test signal ots stored in the memory unit MEM using the evaluation routine BWR. On the basis of the comparison result, the first power controller LR1, controlled by the measurement and evaluation routine MR, BWR, is used

Control unit MC, a first control signal rsl for increasing or possibly reducing the power P _{s of} the optical test signal ots is formed. As a result, the power P _{s of} the optical test signal ots is increased, for example, until a first entry threshold SBSi of the stimulated Brillouin scatter is reached, ie the power P _{ros of} the backscattered signal ros corresponds, for example, to 1/10 of the power P _{s of} the coupled test signal ots. The value of the first critical current currently delivered when the first entry threshold SBSi of the stimulated Brillouin scatter was reached

The power or first SBS entry threshold P _{S1 of} the optical test signal ots is stored in the memory unit MEM in accordance with the measurement routine MR.

According to the invention, in a second step of the measuring method, in addition to the modulated or unmodulated optical test signal ots, at least one modulated optical one Pump signal ops is coupled into the optical fiber OF with a predetermined first pump signal power P _P1 and a first wavelength λl. For this purpose, an optical pump signal ops with a first wavelength λl and additionally the optical pump signal ops with a first modulation frequency vl are amplitude-modulated in the optical pump signal unit PSU, wherein the amplitude modulation can be designed, for example, as a sine, a rectangular or a sawtooth-shaped amplitude modulation.

The modulated optical pump signal ops is emitted at the first output el of the pump signal unit PSU to the first input il of the controllable switching unit S. According to the second step of the measuring method according to the invention, a control signal ss for switching the first input il of the controllable switching unit S to the output e is generated by the measuring routine MR executed in the control unit MC in the switching regulator SR and transmitted to the controllable switching unit S via the control line SL , Following the connection of the optical pump signal ops from the first input il to the output e of the controllable switching unit S, the optical pump signal ops is passed to the optical coupler OK via the second distribution line VL2. With the help of the optical coupler OK, the optical pump signal ops is coupled into the first distribution line VL1 and transmitted to the circulator Z in addition to the optical test signal ots. The circulator Z couples the optical test signal ots and the optical pump signal ops into the optical fiber OF via the third optical distribution line VL3.

The additional coupling in of the modulated optical pump signal ops generates the nonlinear effect of the cross-phase modulation (XPM) in the optical fiber OF and thus causes phase modulation of the optical test signal ots, which widens the frequency spectrum of the optical test signal ots. The broadening of the frequency spectrum of the optical test signal ots initially reduces the The power of the backscattered optical signal ros ab, ie the part of the coupled-in optical test signal ots, which is backscattered or reflected opposite to the coupling direction due to the narrow-band nonlinear effect of the SBS, thus decreases. The backscattered optical signal ros is in turn passed via the circulator Z and via the first feed line ZL1 to the filter input fi of the filter unit FU. The backscattered optical signal ros is filtered in the filter unit FU and the filtered backscattered signal is largely emitted at the filter output fe. The filtered backscattered signal is then largely transmitted to the input i of the measuring transducer MW via the second feed line ZL2 and converted into an electrical signal using the measuring transducer MW. The electrical signal is fed to the control and evaluation unit CU or the evaluation unit AE via the electrical feed line EZL, in which the power P _{ros of} the electrical signal es and thus of the backscattered optical signal ros is determined or evaluated.

As already described, the power unit P _{ros of} the backscattered optical signal ros is determined by the evaluation unit AE controlled by the control unit MC, and the determined backscattered signal power P _{ros is determined} with the power P _{s of} the optical power stored in the memory unit MEM using the evaluation routine BWR Test signal ots compared. On the basis of the comparison result, with the aid of the first power controller LR1, controlled by the measurement and evaluation routine MR, BWR of the control unit MC, the first control signal rsl for increasing the power P _{s of} the optical test signal ots is formed. The power P _{s of} the optical test signal ots is increased until a second entry threshold SBS _{2 of} the stimulated Brillouin scatter, which is higher than the first entry threshold SBSi, is reached, ie the power P _{ros of} the backscattered signal ros in turn corresponds to 1/10 of the power P, for example _{s of} the coupled test signal ots. The value of when the second income Step threshold SBS _{2 of} the stimulated Brillouin scatter currently output second critical power P _s2 or second SBS entry _threshold P _{SBS2 of} the optical test signal ots is stored in the memory unit MEM in accordance with the measurement routine MR. Furthermore, the currently set first optical pump signal power P P is _stored in the memory unit MEM.

FIG. 2 shows an example of the first SBS entry threshold SBSi and the shifted or increased second SBS entry threshold SBS _{2 in a} diagram. The diagram has a horizontal axis (abscissa) and a vertical axis (ordinate), the power P _{s of} the injected optical test signal ots being plotted along the horizontal axis and the power P _{ros of} the backscattered optical signal ros being plotted in dBm along the vertical axis is. To measure the first SBS entry threshold SBSi shown, the optical test signal ots was coupled into the optical fiber OF in accordance with the first step of the measurement method according to the invention, the test signal power P _{s was} increasingly increased and the change or increase in the power P _{ros of} the backscattered optical signal ros was recorded , On the basis of the diagram shown in FIG. 2, the occurrence of the non-linear effect of the SBS becomes clear, which, for example in the case shown, is at a test signal power P _s of approximately 0.002 watts. From this critical test signal power P _s , a significantly faster increase in the measurement curve in the first measurement method step or a significant increase in the power P _{ros of} the backscattered optical signal ros can be seen on the basis of the SBS. This steep rise in the first

SBS entry threshold SBS _X results in a band of test signal power P _s of approximately 2 dBm and then flattens out again, so that the profile of power P _{ros of} the backscattered optical signal ros over the test signal power P _s assumes almost the same slope as immediately before the first SBS entry threshold SBS _X. According to the second step of the measuring method according to the invention, in addition to the optically 1975

20 see test signal ots an optical pump signal ops is coupled into the optical fiber OF, as a result of which the cross-phase modulation XPM occurring in the optical fiber OF shifts the SBS entry threshold to the right, ie the non-linear effect of the SBS occurs at a higher coupled test signal power P _s on. For the measurement curve shown in the diagram and containing the second SBS entry threshold SBS ₂ , for example, an optical pump signal ops was coupled into the optical fiber OF, which was amplitude-modulated with a modulation frequency of 20 MHz and had a pump signal power of 0.2 watts. Furthermore, the wavelength difference Δλ between the optical test signal ots and the optical pump signal ops was approx. 10 nm. The shift in the SBS entry thresholds that can be seen from the diagram is approx. 2 dBm, together with the known test and pump signal parameters and the known fiber parameters for determining the Nonlinearity coefficient γ evaluated. For a clear evaluation of the SBS entry thresholds for the purpose of determining the non-linearity coefficient γ according to the invention, for example, a shift of the SBS entry threshold by 1 to 3 dB is required.

As already indicated in the description part comprising the theoretical basis for understanding the invention, by combining the equations (A-II, A-12) and by forming the ratio for the measured values of the two measurement curves shown in FIG. 2, the 2 dB in the example Comprehensive increase in the SBS entry threshold from SBSi to SBS _2, for example, as a function of the product of the polarization-dependent constant ξ, the non-linearity constant γ and the coupled pump power P _Pi , P _P2 and the product of the dispersion constant D, the wavelength difference Δλ and the Modulation frequency fmod shown. Such an evaluation of the measurement curves shown in FIG. 2 is shown in FIG. 3, for example in a diagram, a third in FIG. 2 not being shown in FIG. the measured curve is evaluated. The diagram shows a first, second and third first, second and third measurement curve MK1, MK2, MK3 determined from the known measurement parameters determined according to the invention. For this purpose, the diagram has a horizontal axis (abscissa) and a vertical axis (ordinate), the product of the polarization-dependent constant ξ, the non-linearity constant γ and the pump power Ppi _/ Pp ₂ ξ * Y being coupled in along the horizontal axis * P _P is plotted on a logarithmic scale and along the vertical axis the product of the dispersion constant D, the wavelength difference Δλ and the modulation frequency fmod D * Δλ * fmod. The measurement curves MK1, MK2, MK3 shown result for a 100 km long optical fiber OF with an attenuation constant of 0.2 dB, the product ξ * γ * P _P on the abscissa having a value range for the pump power P _P of approx. 0.1 to 2 watts and the product D * Δλ * fmod plotted on the ordinate comprises a range of values for the wavelength spacing Δλ around 10 nm at a modulation frequency of 0 to 1 GHz. The first measurement curve MK1 stands for an increase in the first SBS entry threshold SBSi by 1 dB, the second measurement curve for an increase by 2 dB and the third measurement curve for an increase by 3 dB, the pump signal power P _P coupled into the optical fiber for this purpose accordingly increased from 0.1 watts to 0.2 watts. In addition, a first, second, third and fourth measurement point MP1 to MP4 along the second and third measurement curve MK2, MK3 are marked in FIG. 3, which are selected for the determination of the non-linearity constant γ and the dispersion constant D using an iterative evaluation method, for example. For the measurement method according to the invention for determining the non-linearities of the optical fiber OF per se, the determination of at least two measurement values is sufficient. In the illustrated embodiment, however, a more detailed representation of the method according to the invention is preferred. To determine the non-linearity coefficient γ, the measured values stored in the memory unit MEM, first, second test signal power P _S ι, Ps2, first pump signal power P _P1 and the measurement curves shown in FIG. 2 and FIG. 3 are evaluated with the aid of the evaluation routine BWR running in the control unit MC. With a wavelength difference Δλ between the optical test signal ots and the optical pump signal ops around 1 n and a low modulation frequency fmod around 200 MHz, the influence of dispersion on the measurement result is negligible, so that according to equation (A-12) the effective length L _eff in the first place Approximation does not depend on the dispersion constant D and thus the nonlinearity constant γ can be determined by evaluating equation (A-12) with the help of the evaluation routine BWR.

The second measurement curve MK2 shown in FIG. 3, in particular the first measurement point MP1, is used, for example, to explain the determination of the nonlinearity constant γ with the aid of the evaluation routine BWR. The first measurement point MP1 denotes the intersection of the second measurement curve MK2 with the abscissa in FIG. 3, which thus takes into account the negligible dispersion constant D and the small wavelength difference Δλ for the case under consideration. A negligible ordinate value and a logarithmic abscissa value (10 * log10) of -40.9 1 / m / W can thus be read from the diagram in FIG. 3 as coordinates of the first measuring point MP1. The first pump signal power _P ι stored in the memory unit MEM is 20 dBm, which corresponds to a first pump signal power P _P ι of 100 mW. Taking into account the polarization-dependent constant ξ = l, this results in a nonlinearity constant γ of 0.000813 1 / mW after the following transformations:

10 * logl0 (γ * P _P ι) = -40.9 1 / m / W

γ = 0.000813 1 / m / W The non-linearity constant γ can be determined in an analogous manner with the aid of the evaluation routine BWR, for example at the intersection ^{¬ of} the third measurement curve MK3 with the abscissa of the diagram shown in FIG.

Furthermore, according to the invention, the dispersion properties, ie the dispersion constant D, of the optical fiber OF are determined in such a way that in a third step, in addition to the optical test signal ots, the amplitude-modulated optical pump signal ops with the first pump signal power P _P ι and a second Pump wavelength λ2 is coupled into the optical fiber OF and again a third shifted entry threshold SBS _{3 of} the stimulated Brillouin scattering is determined by changing the power of the backscattered optical signal ros by increasing the first pump signal power P _P ι until the power P _{ros of} the backscattered signal ros in turn corresponds, for example, to 1/10 of the power P _{s of} the injected test signal ots. Ie if the wavelength difference Δλ between the optical test signal ots and the second optical pump signal ops2 was increased in the third step, for example from 1 to 10 nm, the first pump power P _PX must be increased by 3 dB in order to again obtain the second SBS entry threshold SBS ₂ . This results in a second optical pump signal power P _P2 with an increase in the wavelength difference Δλ for reaching the second SBS entry threshold SBS ₂ or in other words: due to the increased wavelength difference Δλ between the optical test signal ots and the second optical pump signal ops2, the dispersion has an effect based on the measurement result such that an increase in the first optical pump signal power P _{Pi is} required to reach the second SBS entry threshold SBS ₂ .

This technical effect is evaluated according to the invention to determine the dispersion constant D as follows. Assuming a polarization-dependent constant ξ = 1, the measurement curves shown in FIG. 3 are used, in particular PT / DE01 / 01975

24 the first and third measurement curves MK1, MK3 are used for the determination with the aid of the evaluation routine BWR. In the calculation of the first measurement curve MKl, a first measured pump signal power P _Pι of 20 dBm and in the calculation of the third measurement curve MK3, a third measured pump signal power P _P of 26 dBm was evaluated, which increases the pump power P _P by 6 dB to compensate for the Increasing the wavelength difference Δλ corresponds, that is, in order to achieve the second SBS entry threshold SBS ₂ , a first pump signal power P _{Pi is} required, for example, in the case of a first optical pump signal opsl with a first pump wavelength λl and when using a second optical pump signal ops2 with an increased second pump wavelength λ2 a second pump signal power P _P2 increased by 6 dB is required. The third measurement curve MK3 thus obtained, which is shifted to the right in the diagram compared to the first measurement curve MK1, is evaluated starting at the second measurement point MP2 to determine the dispersion constant D with the aid of an iterative evaluation method. For this purpose, the data record representing the third measurement curve MK3 and stored in the memory unit MEM is evaluated with the aid of the evaluation routine BWR in such a way that the intersection point between the abscissa and the third measurement curve MK3, the second measurement point MP2 is selected and starting from the abscissa value of the second Measuring point MP2, the abscissa value of the fourth measuring point is determined from the data set by the abscissa value of the second measuring point MP2 being shifted to the right or decreased by the amount of the increase in the pump signal power P _P , in the exemplary embodiment under consideration by 6 dB. Based on this, the associated ordinate value of the fourth measuring point MP4 is determined.

Based on the difference between the first product of the dispersion constant D, the first wavelength difference Δλl and the modulation frequency fmod D * Δλl * £ od for the first measurement curve MKl and the second product of the dispersion constant D, the second wavelength difference Δλ2 and the modulation frequency fmod D * Δλ2 * fmod for the third measurement curve MK3 of 10, the first measurement point MPl or starting point of the iterative evaluation method was not precise enough and is improved as follows. The previously determined ordinate value of the fourth measuring point MP4 is divided by the difference between the first and second product, the factor 10, and thus a new improved ordinate value for the first measuring point MPl is determined. For the new improved ordinate value, the associated new, improved abscissa value of the first measuring point MP1 is determined on the basis of the database and stored in the memory unit MEM for further processing. According to the first iteration step, the new improved abscissa value of the second measurement point MP2 is again shifted or decreased by the amount of the increase in the pump signal power P _P , in the exemplary embodiment under consideration by 6 dB, in a second run of the iterative evaluation method. Based on this, an improved ordinate value of the resulting new fourth measuring point MP4 is determined. In most cases, this evaluation method converges after a few iterations, so that the ordinate value obtained for the fourth measuring point MP4 can be used to determine the dispersion constants in accordance with the following equation:

D = 4.4 * 10 ^"4 / (Δλ * fmod)

4.4 * 10- ⁴ / (10 ^"8 * 2 * 10 ⁸ ) ps / nm / km = D = 220 ps / nm / km

This results in a dispersion constant of D = 220 ps / nm / km for the exemplary embodiment shown. The second measurement curve for determining the associated dispersion constant D can be evaluated in an analogous manner.

The measuring arrangement according to the invention is in no way limited to a transmission-side implementation, but can be used for bige optical transmission media can also be used at the receiving end.

Claims

claims

1. Measuring method for determining the nonlinearities in an • optical fiber (OF), characterized in that in a first step at least one optical test signal (ots) is coupled into the optical fiber (OF), the test signal power (P _s ) is changed and on the basis of the change in the power (P _ros ) of the backscattered optical signal (ros), a first entry threshold (SBSi) of the stimulated Brillouin scatter is determined, that in a second step at least one modulated optical pump signal in addition to the optical test signal (ots) (ops) is coupled into the optical fiber (OF) with a predetermined pump signal power (P _Pi ) and a first pump wavelength (λi) and a second entry threshold (SBS ₂ ) of the stimulated Brillouin scattering is determined on the basis of the change in the test signal power (P _s ) and that by evaluating at least the first and second entry thresholds (SBSi, SBS ₂ ), the test and pump signal parameters and the fiber parameter, the nonlinearity coefficient (γ) of the optical fiber (OF) is determined.

2. Measuring method for determining the non-linearities in an optical fiber, characterized in that in a first step at least one optical test signal (ots) with a test signal power (P _s ) and a test signal wavelength (λi) is inserted into the optical fiber (OF). is coupled and the power (P _ros ) of the backscattered optical signal (ros) is measured and a first ratio of the coupled test signal power (P _s ) and the power (P _ros ) of the backscattered optical signal is formed, that in a second step in addition to the optical test signal (ots) having a test signal power (P _s ) and test wavelength (λi) at least one modulated optical pump signal (ops) with adjustable pump signal power (P _P ι) 0101975

28 and a first pump wavelength (.lambda..sub.i) into the optical fiber (OF) is coupled, and the power (P _ro s) of the rückge ^¬ scattered optical signal (ROS) is measured and a second ratio of launched test signal power (P _s) and the performance of the backscattered optical signal (P _ros ) is determined that the adjustable pump signal power (P _Pi ) of the modulated optical pump signal (ops) is increased or decreased until the second ratio matches the first ratio and that by evaluating the test and pump signal parameters and the fiber parameter, the non-linearity coefficient (γ) of the optical fiber (OF) is determined.

3. Measuring method according to claim 1, characterized in that the test signal wavelength (λi), the predetermined pump signal power (P _P ι), the first pump wavelength (λi) and the modulation frequency (υi) of the optical pump signal (ops) are evaluated as test and pump signal parameters ,

4. Measuring method according to claim 2, characterized in that as the test and pump signal parameters the test signal power (P _s ), the test signal wavelength (λi), the set pump signal power (PP _I ), the first pump wavelength (λi), the modulation frequency (υi) of the optical Pump signals (ops) can be evaluated.

5. Measuring method according to claim 3 or 4, characterized in that the test signal wavelength (λ _x ) and the first pump wavelength (λi) have a wavelength difference less than 1 nm.

6. Measuring method for determining the dispersion (D) of an optical fiber (OF) according to claim 1 and 3, 01 01975

29 characterized in that in a third step, in addition to the optical test signal (ots), the modulated optical pump signal (ops) with a predetermined pump signal power (P _P ι) and a second pump wavelength (λ ₂ ) is coupled into the optical fiber (OF) and again a third shifted entry threshold (MK3) of the stimulated Brillouin scatter is determined by a change in the power (P _ros ) of the backscattered optical signal (ros), and that the additional evaluation of at least the second and third entry threshold (SBS _X , MK3), the test wavelength (λi), the pump signal power, the first and second pump wavelength (λ _ι , λ ₂ ), the modulation frequency (Ό _X ) of the optical pump signal (ops) and the fiber parameters, the dispersion constant (D) of the optical fiber (OF) is determined.

7. Measuring method for determining the dispersion (D) in an optical fiber (OF) according to claim 2 and 4, characterized in that in a third step in addition to the test signal power (P _s ) and test signal wavelength (λ _x ) having optical test signal ( ots) the modulated optical pump signal (ops) with an adjustable pump signal power (Ppi) and a second pump wavelength (λ ₂ ) is coupled into the optical fiber (OF) and the power (P _rθs ) of the backscattered optical signal (ros) is measured and a third ratio of the injected test signal power (P _s ) and the measured power (P _ros ) of the backscattered optical signal (ros) is determined that the adjustable pump power (P _P ι) of the modulated optical pump signal having a second pump wavelength (λ ₂ ) (ops) is increased or decreased until the third ratio matches the first ratio and that by an additional one Evaluation of the test signal power (P _s ), the test signal wavelength (λi), the set pump signal power (Ppi), the second pump wavelength (λ ₂ ), the modulation frequency (υ _x ) of the optical pump signal (ops) and PT / DE01 / 01975

30 of the fiber parameters, the dispersion constant (D) of the optical fiber (OF) is determined.

8. Measuring method according to one of claims 1 to 7, characterized in that the fiber parameters effective fiber length (L _eff ), attenuation constant (α), polarization factor (ξ) with randomly varying polarization and Brilloin gain factor (g _B ) of the optical fiber (OF) be taken into account in the evaluation.

9. Measuring method according to claim 1 to 8, characterized in that the first, second and third entry threshold (SBSι, SBS ₂ , MK3) of the stimulated Brillouin scatter are each determined by the coupled test signal power (P _s ) causing the entry of the stimulated Brillouin effect.

10. Measuring method according to one of claims 1 to 9, so that the modulation of the optical pump signal (ops) is realized by a sinusoidal, rectangular or sawtooth-shaped amplitude modulation.

11. Measuring method according to one of claims 1 to 10, characterized in that an evaluation according to the formulas

and L _ef f (z, α, γ, I _P , D, Δλ, fmod) = 1 / α * (1-exp (-α * z / n)) *

"-1

^• {1+ Σ J exp (-α * k / n * z) * J ₀ ² (m (kz / n) ^* Le (k * z / n, α, D, Δλ, fmod))}

A = l with

with: L = k ^* z / n, ß = D * Δλ, ω = 2 * π * fmod; m (kz / n) = ξ * γ * I _P * Le;

done by

P _s ^or = the backscattered power at the entry threshold of the SBS, g _B = the Brillouin Gain constant,

A _eff = the effective area,

Le _ff = the effective length, z = the location variable, α = the fiber attenuation constant, D = the dispersion constant,

Δλ = the wavelength difference between the test and pump signals, fmod = the modulation frequency of the pump signal,

I _P = the pump power of the injected pump signal, n = the number of sections for the approximation, γ = the non-linearity coefficient, ξ = a polarization-dependent constant.

12. Measuring method according to claim 6 or 7, characterized in that the modulation frequency (υ _x ) of the optical pump signal

(ops) is chosen to be larger than the SBS line width (Δυ _B ).